The present disclosure relates to a developing device that uses a toner to develop an electrostatic latent image and an image forming apparatus that includes such a developing device. The present disclosure also relates to a method of controlling a developing device.
In some of image forming apparatuses such as a multifunctional peripheral, a copying machine, a printer and a facsimile machine, a toner is used to develop and print an electrostatic latent image. In some of image forming apparatuses, a developer (so-called two-component developer) is used that contains a carrier formed with a magnetic material and a toner. When a two-component developer is used, it is not preferable, in terms of image quality and the like, to bring the magnetic brush produced by a carrier into direct contact with a photoconductor drum. Hence, an image forming apparatus is provided that has a developing device using a method (also referred to a “touchdown development” or a “hybrid development”) in which a development roller is provided opposite the photoconductor drum to bear the toner, the magnetic brush is formed with a magnetic roller opposite the development roller, the magnetic brush transfers only the toner to the development roller and an electrostatic latent image is developed without the magnetic brush being pressed onto the photoconductor drum. This method is advantageous, in various respects such as image quality, a printing speed, the life of the toner and the prevention of scattering of the carrier, over a one-component development method or a conventional two-component development method.
For example, an image formation method and an image forming apparatus are known in which a development roller that forms a thin layer of a toner on a surface thereof and a magnetic roller that feeds, with a magnetic carrier, the toner to the development roller are used, and in which electrostatic latent images are developed one after another with the development roller to perform image formation on sheets.
In the touchdown development method as described above, an alternating-current voltage (having a peak-to-peak voltage of, for example, about 1 to 2 kV) is applied to the development roller. Then, the charged toner is blown from the development roller to develop the electrostatic latent image. It is likely that a switching element such as a transistor is used to input signals indicating the turning on and off of energization to a transformer, and that an alternating-current voltage which is applied to the development roller is obtained.
Here, it is likely that, in order to prevent the occurrence of a leak between the photoconductor drum and the development roller (to prevent the occurrence of an electrical discharge) and prevent the occurrence of unevenness in the toner image, it is desirable to be able to change a duty ratio in the switching according to the state of the image forming apparatus (according to the mode). However, the change of the duty ratio in the switching causes an unbalanced voltage (a voltage whose energy is unevenly distributed) to be applied to the transformer, and this results in the occurrence of asymmetric magnetization in the transformer.
Then, when the asymmetric magnetization occurs to displace a magnetic flux, the transformer is brought into a state where the transformer appears to be biased by a direct current. A current (overcurrent) higher than a rating is passed, and thus it is more likely that the switching element is damaged. In particular, as the amount of instantaneous change in the duty ratio is increased, a larger amount of asymmetric magnetization occurs. For example, an alternating-current voltage is applied to the transformer with the asymmetric magnetization occurring, and thus magnetic saturation occurs. Hence, the impedance of the transformer is significantly reduced, with the result that a relatively large current may flow through the switching element. Therefore, when the duty ratio is changed, it is disadvantageously necessary to prevent the passage of a current large enough to cause the damage of the switching element.
As a conventionally known developing device, the fast-speed small hybrid developing device described above is present. However, no consideration is given to the possibility that, when the duty ratio is changed, a large current flows through the switching element. Therefore, it is impossible to solve the above problem with the conventional technology.